Nano-electro-mechanical DRAM cell

ABSTRACT

A DRAM cell and method for storing information in a dynamic random access memory using an electrostatic actuator beam to make an electrical connection between a storage capacitor and a bit line.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT

None.

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

None.

FIELD OF THE INVENTION

The invention disclosed broadly relates to the field of dynamicrandom-access memory (DRAM) cells and more particularly relates to thefield of nano-electro-mechanical dynamic random-access memory (NEM-DRAM)cells.

BACKGROUND OF THE INVENTION

FIG. 1 shows an exemplary 1T1C (one-transistor, one capacitor) DRAM cell100. This device includes a MOSFET pass-gate 102 and a storage capacitor104 coupled therewith. The storage capacitor 104 uses either a trenchtechnology or a stacked capacitor structure. The cell can be bulk-Si orSilicon on Insulator (SOI). This is currently the cell technology ofchoice for dense memory in both embedded and standalone applications.DRAM technology, however, inherently has a need for a non-negligiblestandby power supply due to the need to periodically refresh storeddata. This is fundamental in a 1T1C cell since the current leakage ofthe pass-gate device in the DRAM cell is non-zero due to subthreshold,junction, and gate leakage currents.

As device dimensions are scaled, these currents inevitably increase dueto short-channel effects, band-to-band tunneling, and gate oxidetunneling. Thus, especially in scaled technologies, standby powerreduction in conventional DRAM is very difficult. With technologyscaling, variability increases, which compounds these problems. In alarge memory array, the refresh rate is limited by the cell with thelowest Vt (voltage) pass-gate while the performance is limited by thecell with the highest Vt pass-gate. With variability, the nominal deviceVt must be very high to ensure that retention targets can be met. Thisrequires very high channel doping, which, in turn, increases junctionleakage and dopant-fluctuation-induced Vt variation.

Variability also means that the gate voltage (often charge-pumped tocompensate for the Vt drop when the NFET pass-gate charges up thestorage capacitor) on the pass-gate, such as word line (WL) highvoltage, must be increased to maintain performance. This max voltage isnow approaching fundamental limits in oxide breakdown characteristics.

Since VLSI technology is subject to power constraints, methods to reducestandby power are especially important. Reduced power benefitsapplications ranging from high performance (e.g. the amount of cachethat can be added to a server is often limited by power dissipation) tolow power (e.g. standby power for cellular phones determines batterylife). Going forward, variability also limits DRAM scaling, whichdirectly leads to tradeoffs in performance and/or power dissipation.

A mechanical memory cell has been proposed in the past, but this wastargeted towards non-volatile memory applications and suffers from largecell size due to the need for multiple cantilever beams per cell. SingleDRAM cell functionality based on mechanical actuation of carbonnanotubes has also been demonstrated, but the cell design is inadequatefor efficient actuation of the cantilever beam (voltages of ˜15V werenecessary) and relies upon un-established devices in the form of carbonnanotubes, which cannot be applied to, for example, conventional trenchcapacitor structures.

SUMMARY OF THE INVENTION

Briefly, according to an embodiment of the invention a method comprisessteps or acts of coupling a cantilever beam to a bit line of a memoryarray or storage node of a cell, wherein the cantilever beam is orientedparallel to a wafer substrate and is actuated upwards to make electricalconnection between the storage node of the cell and the bit line;electrically connecting an electrostatic actuator to the word line ofthe memory array; and activating the cell by applying a high voltage toinduce electrostatic pull-in of a relay to perform either a read orwrite operation.

According to another embodiment, a method comprises steps or acts of:coupling a cantilever beam to a bit line of a memory array or storagenode of a cell, wherein the cantilever beam is oriented parallel to awafer substrate and is actuated laterally to make electrical connectionbetween the storage node of the cell and the bit line; electricallyconnecting an electrostatic actuator to the word line of the memoryarray; and activating the cell by applying a high voltage to induceelectrostatic pull-in of a relay to perform either a read or writeoperation.

According to yet another embodiment, a method comprises steps or acts ofcoupling a cantilever beam to a bit line of a memory array or storagenode of a cell, wherein the cantilever beam is oriented perpendicular toa wafer substrate and is actuated from the side to make electricalconnection between the storage node of the cell and the bit line;electrically connecting an electrostatic actuator to the word line ofthe memory array; and activating the cell by applying a high voltage toinduce electrostatic pull-in of a relay, thereby electrically connectingthe storage node of the cell to the bit line to perform either a read orwrite operation.

BRIEF DESCRIPTION OF THE DRAWINGS

To describe the foregoing and other exemplary purposes, aspects, andadvantages, we use the following detailed description of an exemplaryembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a schematic of a conventional DRAM cell;

FIG. 2 shows two potential circuit schematics for a NEM-DRAM cell,according to an embodiment of the present invention: FIG. 2A shows thecantilever beam tied to the bit line and FIG. 2B shows the cantileverbeam tied to the storage node;

FIGS. 3A and 3B show possible potential configurations for activated andinactivated cells of state “1” and “0,” for a write and a readoperation, respectively, according to an embodiment of the presentinvention;

FIG. 4A shows the device layer of a basic implementation of a NEM-DRAMcell using downward actuation, according to an embodiment of the presentinvention;

FIG. 4B show the wiring layout of a basic implementation of a NEM-DRAMcell using downward actuation, according to an embodiment of the presentinvention; and

FIG. 4C shows the cross-section of the layout of FIG. 4A, according toan embodiment of the present invention;

FIG. 5 shows two layouts in which the exemplary cell of FIG. 4A can betiled out into an array, according to an embodiment of the presentinvention;

FIGS. 6A through 6C show different views of a cell design that orientsthe cantilever beam parallel to the wafer substrate with upwardactuation, according to an embodiment of the present invention;

FIGS. 7A through 7D show different views of a cell design that actuatesa cantilever beam laterally, according to an embodiment of the presentinvention;

FIGS. 8A through 8C show different views of a cell design that orientsthe cantilever beam parallel to the wafer substrate with downwardactuation that is anchored to the cell storage node, according to anembodiment of the present invention;

FIGS. 9A through 9C show different views of a cell design that orientsthe cantilever beam parallel to the wafer substrate with upwardactuation that is anchored to the cell storage node, according to anembodiment of the present invention;

FIGS. 10A through 10D show different views of a cell design that orientsthe cantilever beam parallel to the wafer substrate with lateralactuation that is anchored to the cell storage node, according to anembodiment of the present invention;

FIGS. 11A through 11C show different views of a cell design that uses avertical beam orientation, according to an embodiment of the presentinvention;

FIGS. 12A-N show the physical structure of the basic memory cellstructure as it is fabricated according to the invention:

FIG. 12A shows a shallow trench isolation (STI) starting from an STImodule of an eDRAM (embedded DRAM), post pad nitride strip;

FIG. 12B shows depositing poly during fabrication of the gate stackmodule and pattern using standard PC module processing;

FIG. 12C shows completion of the FEOL (front end of the line);

FIG. 12D shows the MOL (middle of line) part of the process;

FIG. 12E shows a step of actuator formation;

FIG. 12F shows a contactor formation;

FIG. 12G shows an oxide deposition;

FIG. 12H shows a step of anchor point definition;

FIG. 12I shows a step of cantilever metallization;

FIG. 12J shows a step of cantilever patterning;

FIG. 12K shows a step of cantilever release;

FIG. 12L shows a step of encapsulation bubble;

FIG. 12M shows a step of bubble sealing;

FIG. 12N shows a step of bit line metallization; and

FIG. 13 shows a flowchart of a fabrication method according to anembodiment of the present invention.

While the invention as claimed can be modified into alternative forms,specific embodiments thereof are shown by way of example in the drawingsand will herein be described in detail. It should be understood,however, that the drawings and detailed description thereto are notintended to limit the invention to the particular form disclosed, but onthe contrary, the intention is to cover all modifications, equivalentsand alternatives falling within the scope of the present invention.

DETAILED DESCRIPTION

We describe a nano-electro-mechanical (NEM) relay (i.e., a switch) asthe pass-gate in a DRAM cell. Such a device has effectively zerooff-current. This eliminates the dominant leakage mechanism in a DRAMcell and could yield improvements in cell retention time by many ordersof magnitude over known designs, thereby reducing DRAM standby power(due to refresh) by orders of magnitude. This enables dramaticimprovements in DRAM power dissipation for a wide range of applications.

The DRAM structure has an actuating gate electrode separate from avertical cantilever beam. The structure is a DRAM cell that depends oncharge storage using a capacitor to hold state (does not depend onstiction at all, which can be very difficult to control), except thatthe pass-gate is formed using a mechanical switch. In our structure, theactuating gate exerts a direct electrostatic force on a single verticalcantilever beam, which moves it towards the gate to close the switch. Toachieve sub-1V operation, the rough dimensions that are needed dependheavily on the Young's modulus of the beam material. For silicon, weneed both the beam thickness and gap to be in the 10 nm (or below)range.

In addition, variability in a NEM relay-based DRAM cell can be less ofan issue than in a conventional MOSFET-based DRAM cell, shown in FIG. 1.While the pull-in/pull-out voltages for a NEM relay will be affected bycantilever beam and gap thickness variation, the actual on- andoff-currents in a NEM relay are relatively immune to variation. Thus, aslong as proper voltage margins are maintained to contain thepull-in/pull-out voltages, variability has only a minor impact onNEM-DRAM. In contrast, variation affects all characteristics (V_(t),I_(on), I_(off) of a MOSFET, thus limiting all DRAM cell specs.

We discuss cell designs that can enable area-efficient DRAM cell designsbased on NEMS relays that can be practically combined with conventional(manufacturable) DRAM processes.

FIG. 2A shows a potential circuit schematic for a NEM DRAM cell 200 withthe cantilever beam 206 tied to the bit line (BL) 214. To perform thefunction of a pass-gate device, the cantilever beam 206 can either beelectrically connected to the bit line (BL) 214 of the memory array orthe storage node 208 of the cell (as shown in FIG. 2B). An electrostaticactuator 204 is vertically electrically connected to the word line (WL)212 of the memory array and coupled with the storage capacitor 208. Toactivate the memory cell 200, a high voltage, V_(pp), is applied to theWL 212, which induces electrostatic pull-in of the NEM relay 202,thereby electrically connecting the storage node 208 of the cell 200 tothe BL 214 to perform either a read or write operation. Example voltagesinclude: V_(pp)=1.8V, V_(dd)=0.5V, V_(contact), V_(pull-out)˜1V. Thecell 200 is in standby when differential voltage is less than thevoltage pull-out applied between the BL 214 and the WL 212.

FIG. 2B illustrates a second option showing a cell 250 where thecantilever beam 256 is tied (coupled) to the storage capacitor 258.During a write operation, the bit line BL 214 would be set to either 0or Vdd, depending on data. During a read operation, the BL 214 is bepre-charged to a pre-determined level in the 0-Vdd range (e.g., Vdd),which could then be charged or discharged through charge sharing with anactivated cell. The actuator 254 is vertically connected to the WL 212.

In an inactivated memory cell, the WL 212 is biased to 0, which ensureselectrostatic pull-out of the relay 202, thereby electrically isolatingthe storage node 258 of the cell 250 from the BL 214. In this state, therelay 202 is open, and the leakage current is effectively zero, whichminimizes the need for cell refresh. Implicitly, the operation asdescribed above assumes some constraints on V_(pp) and V_(dd). Here thecantilever beam 256 is coupled (tied) to the storage node (capacitor258). This cell 250 is in standby mode when the differential voltage isless than Vpull-out applied between the storage node 258 and the wordline (WL) 212.

FIGS. 3A and 3B show possible potential configurations for activated andinactivated cells of state “1” and “0.” FIG. 3A shows a write operation300 while FIG. 3B shows a read operation 350. The DRAM cell in theactive state 308 shows the cantilever beam 306 in a closed positionwherein it connects to a storage capacitor 308. The cantilever beam 306extends from the bit line (BL) 314 to make contact with the storagecapacitor 308.

The DRAM cell in the inactive state 320 shows the cantilever beam 306 inthe open position wherein it does not make contact with the storagecapacitor 308. The BL and storage node 308 potentials are data dependentand could each be either BL=0 324 or BL=Vdd 314. Since we desire thatthe WL 322 controls pull-in of the relay, these potentials should notaffect the relay. The constraints are as follows: a) so that theinactive WL 322 pulls out, the V_(dd) should be chosen to be lower thanthe pull-out voltage, V_(pull-out), of the cantilever beam 306 (which isdetermined by beam and gap dimensions and material constants); and b) sothat the active WL 312 pulls in, V_(pp)−V_(dd) should be greater thanV_(pull-in).

If the off-state potential of the WL 312 is also in the 0-V_(dd) range,then it can be ensured that the relay is open and that the cell 320 isin the standby state. When the WL 312 is set to V_(pp), then the relayshould be closed. To ensure this, V_(pp)-V_(dd) (since the potential ofthe beam could be at V_(dd)) must be larger than the pull-in voltage,V_(pull-in), of the cantilever beam. In a cantilever beam system,V_(pull-in) is larger than V_(pull-out); as a result, it is practical toexpect that V_(pp)>V_(dd). This is analogous to a conventional DRAMarray design, in which a charge-pumped V_(pp)>V_(dd) is used to activatethe WL.

For the read operation 350, both BL 314 and BL 324 are precharged toVdd, but the BL 324 discharges slightly (triggers sense amp). Dependingon the specific design parameters, electrostatic pull-in is notessential for device operation—it is only the motion of the cantileverbeam that is needed to open and close the relay switch. Switching of aNEM relay can be on the order of ˜1 nano second when practical materialsand device dimensions are considered. Since the on-current of a NEMrelay can be quite high, the actual charging and discharging of thestorage capacitor can be quite fast.

Thus, the time needed to physically move the cantilever beam 306 islikely to dominate the read/write access latency of a NEM-DRAM cell.Such switching times are acceptable for even high-performance DRAMapplications. Since the leakage current of a NEM relay pass-gate isessentially zero, it eliminates one constraint on the minimum cellcapacitance. This could allow for the use of smaller storage capacitorsand thus ease fabrication (e.g., shallower trench capacitor, thickercapacitor dielectric to further reduce leakage, and so forth).

With a NEM relay pass-gate, the number of bits sharing a single BL canbe increased, thus improving array efficiency (i.e., percentage of arraymacro area occupied by cells). This is because the zero leakage atcurrent eliminates noise margin concerns to BL leakage, and also becausethe on-resistance of a NEM relay can be lower than that of a MOSFET,which enables fast read access despite the higher BL capacitance due tothe increased BL length.

A basic implementation of a NEM-DRAM cell showing downward actuation isshown in FIGS. 4A, 4B, and FIG. 4C. The diagram (and all subsequent celllayouts/cross-sections) assumes a trench capacitor 408 structure, but itshould be apparent to one skilled in the art how the techniquespresented here can be applied to a stacked capacitor structure. Thespecific process flows and design layers (both in material and potentiallayer sharing with integrated CMOS) by which these structures can befabricated vary tremendously. We provide sketches of the final structureand do not wish to limit the structure to specific methods offabrication.

FIG. 4A shows a basic cantilever beam 406 oriented parallel to the wafersubstrate 422 that is actuated from below to make the electricalconnection to a trench capacitor 408. A cell 400 includes an anchor 404,a beam 406, a WL 414 and a capacitor 408. In this embodiment the beam406 is a horizontal structure that when activated moves down to makecontact with the capacitor 408. A via 425 can be used to make contact tothe beam 406 and to route the BL 414 in a separate layer so that it isperpendicular to the WL 412. FIG. 4A shows the device layers; FIG. 4Bshows the wiring layers, and FIG. 4C shows the cell layout incross-section.

FIG. 5 shows two array layouts in which the exemplary cell of FIG. 4 canbe tiled out into an array. FIG. 5A shows that symmetric tiling in thevertical direction can be used to horizontally and vertically translatethe cell. FIG. 5B illustrates mirror-image tiling in the verticaldirection. Mirror-image tiling can be used to share the anchor for thecantilever beam, which can reduce cell area. This does, however, reducetrench capacitor pitch, which may be more difficult to fabricate.

FIG. 6 shows a cell design with upward actuation that again orients thecantilever beam 606 parallel to the wafer substrate 622, but is actuatedfrom above to make contact to a conducting layer tied to the trenchcapacitor 608. FIG. 6A shows the device layers of the cell; FIG. 6Bshows the wiring layers through the via 625; and FIG. 6C shows thecross-section view. Due to additional spacing required between the beam606 and the trench capacitor 608, this cell size may be somewhat largerthan that shown in FIG. 4. However, such a structure may be morecompatible with traditional CMOS processes because one possibleimplementation could be to use an SOI layer for the beam and a MOSFETgate layer for the actuator.

FIGS. 7A through 7D show different views of a NEM-DRAM cell design withlateral actuation that actuates the cantilever beam 706 (again orientedparallel to the wafer substrate 722) laterally. FIG. 7A shows the celllayout. FIG. 7B shows the cross-section through the relay landing pad704. FIG. 7C shows the cross-section through the via 725. FIG. 7D showsthe cross-section through the beam 706. The actuator 702 can be placedadjacent to the beam 706 to make lateral electrical contact to thetrench capacitor 708. This requires wiring of the WL 712 signal in aseparate layer so that it may run perpendicular to the BL 714.

FIGS. 8, 9, and 10 show cell structure designs that can be derived fromthe cell designs in FIGS. 4, 6, and 7. The basic cell arrangement inthese cells differ from the arrangements shown in FIGS. 4, 6, and 7primarily in the anchor point of the cantilever beam. FIGS. 8A, 8B, and8C show different views of the cell layout of a cell design 800 withdownward actuation of the beam 806. FIGS. 8A, 8B, and 8C show the devicelayers of the cell layout, the wiring layers, and the cell layoutcross-section, respectively, of the cell design 800. The anchor point804 of the cantilever beam 806 is placed to make electrical contact withthe trench capacitor 808 instead of the BL 814.

FIGS. 9A through 9C show different views of the cell design with upwardactuation of the beam 906. FIGS. 9A, 9B, and 9C show the device layers,the wiring layers, and the cell layout cross-section, respectively, ofthe cell design 900. The anchor point 904 of the cantilever beam 906 isplaced to make electrical contact with the trench capacitor 908 insteadof the BL 914.

FIGS. 10A through 10D show different views of a NEM-DRAM cell design1000 with lateral actuation of the beam 1006. The cell layout 1000 isshown in FIG. 10A. FIG. 10B shows a cross-section view through thetrench capacitor 1008. FIG. 10C shows a cross-section through the via1025. FIG. 10D shows a cross-section view through the beam 1006. Notethat the beam 1006 in this embodiment is parallel to the wafer substrate1022.

FIGS. 11A through 11C show different views of a NEM-DRAM cell design1100 that uses a vertical beam 1106 orientation (perpendicular to thewafer substrate 1122) to create a NEM-DRAM cell 1100 with the smallestpossible areal footprint. FIG. 11A shows the device layers of the celllayout 1100. FIG. 11B shows the wiring layer of the cell 1100. FIG. 11Cshows a cross-section view of the cell layout 1100. The beam 1106 is incontact with the trench capacitor 1108. The cell size could potentiallybe as small as a 6F² DRAM design, with a length of approximately 3F anda width of 2F. This size, in addition to providing dramatic standbypower reduction, is smaller than many conventional DRAM cells.

FIGS. 12A-N shows the physical structure of the basic NEM-DRAM cell ofFIG. 4 during fabrication, according to an embodiment of the presentinvention. These figures focus on CMOS (complementary metal-oxidesemiconductor) integration of a vertical gap cell.

FIG. 12A shows a silicon substrate after formation of the shallow trenchisolation (STI) cap 1202 as might be used in a standard SOI eDRAM(embedded DRAM) process. The trench capacitor 1202 structure has alreadybeen formed according to standard techniques. The structure is shownpost pad nitride strip.

FIG. 12B shows WL formation by depositing polysilicon (poly) duringfabrication of the gate stack module and patterning using standard PCmodule processing. Using polysilicon (poly) for the WL 1212 allows for adenser cell and easier process than otherwise using the SOI active layer(RX).

FIG. 12C shows completion of the FEOL (front end of the line) as wouldbe used for standard MOSFET fabrication with spacers 1230, and implants.This requires proceeding through FEOL as normal up through the silicidemodule.

FIG. 12D shows the MOL (middle of line) stage of the process. Thisrequires depositing an oxide and nitride stack, performing contact (CA)patterning by lithography, RIE (reactive ion etching) and tungsten (W)metallization 1240. The nitride must be high quality and low temperature(e.g. sputtered SiN).

FIG. 12E shows the actuator formation step. This requires depositing ablanket metal layer 1250 (e.g. platinum) with sputtering and patternwith lithography and ion milling. The contact to the WL 1212 should beas close as possible to the capacitor contact.

FIG. 12F shows the contactor formation step. This requires depositingblanket metal (e.g. platinum) and a thin SiO₂ layer pattern 1260 withlitho and ion milling. The SiO₂ layer is preferably between 5 and 10 nm.

FIG. 12G shows a conformal oxide deposition step to define the gapbetween the WL electrode and the BL cantilever beam 1206. The SiO₂ 1270is deposited conformally, at a thickness of approximately 50 nm.

FIG. 12H shows the anchor point definition step. This requirespatterning contact holes in the top oxide layer to open an anchor point1204 and to form an electrical connection 1280 to the contactor 1282.

FIG. 12I shows the cantilever metallization step to deposit thecantilever material 1285. This requires depositing metal or metalmultilayer to achieve a “zero stress” layer 1285.

FIG. 12J shows the cantilever patterning step. This requires lithographyand patterning using either dry etching or ion milling of the cantilevermaterial 1285.

FIG. 12K shows the cantilever release step. This requires sacrificialoxide removal using HF (hydrogen fluoride) etching and super criticaldrying. It is important to note that the gap between the contactor 1282and the cantilever beam 1206 is smaller than the gap between theactuator 1204 and the cantilever beam 1206.

FIG. 12L shows the formation of an encapsulation bubble 1288, which canbe performed by using resist as a sacrificial material. Low temperaturePECVD (Plasma Enhanced Chemical Vapor Deposition) oxide can be depositedover the resist, lithography and RIE can be used to open release holesin the oxide, and dry etching (which avoids stiction issues) can be usedto remove the resist.

FIG. 12M shows the bubble sealing step of the process wherein theencapsulation bubble is sealed. Sealing of the holes used to remove thesacrificial resist requires using sputtered oxide followed by PECVDoxide deposit 1292 and planarization as in a standard BEOL process.

FIG. 12N shows the BL 1214 metallization step. This requires proceedingwith standard back-end-of-line (BEOL) metallization 1298 to form the BL1214.

FIG. 13 is a flowchart of the fabrication steps as described above. Theprocess begins at step 1310 with an STI module, then proceeds with thedeposition and patterning of poly-silicon in step 1312. Next at step1314 we perform an FEOL build through the silicide, followed byfabrication of the actuator 1250 in step 1316. In step 1318 thecontactor 1282 is formed, followed by oxide deposition at step 1320.

Next, in step 1322 we open an anchor point 1204 and contact to the topPt layer, followed by metallization and forming of the cantilever 1206at step 1324. The cantilever 1206 is then patterned in step 1326. Afterpatterning, the cantilever 1206 is released in step 1328. Anencapsulation bubble 1288 is formed over the module in step 1330, afterwhich the bubble 1288 is sealed in step 1332. BEOL metallizationcompletes the process at step 1334.

Therefore, while there has been described what is presently consideredto be the preferred embodiment, it will understood by those skilled inthe art that other modifications can be made within the spirit of theinvention. The above descriptions of embodiments are not intended to beexhaustive or limiting in scope. The embodiments, as described, werechosen in order to explain the principles of the invention, show itspractical application, and enable those with ordinary skill in the artto understand how to make and use the invention. It should be understoodthat the invention is not limited to the embodiments described above,but rather should be interpreted within the full meaning and scope ofthe appended claims.

We claim:
 1. A dynamic random-access memory (DRAM) structure comprising:a word line; a bit line; and a nano-electro-mechanical (NEM) relaycomprising: a cantilever beam; a storage capacitor; and an electrostaticactuator gate coupled with the storage capacitor and verticallyelectrically connected to the word line, wherein said electrostaticactuator is separate from the cantilever beam and applies a high voltageto cause the cantilever beam to electrically connect the storagecapacitor to the bit line to perform either a read or write operation.2. The structure of claim 1 wherein the electrostatic actuator causesthe cantilever beam to connect the bit line to the storage capacitorwhen actuated.
 3. The structure of claim 2 wherein, during a readoperation the bit line is set to a predetermined level in the zero toV.sub.dd range and then used to charge or discharge the storagecapacitor in an activated cell.
 4. The structure of claim 3 wherein,during a write operation, the bit line is first set to zero volts towrite a logical zero or Vdd to write a logical one.
 5. The structure ofclaim 2 wherein the cantilever beam is coupled to the bit line and movesfrom a first position wherein said cantilever beam does not makeelectrical contact with the storage capacitor to a second positionwherein said cantilever beam makes contact with the storage capacitor,responsive to a pull-in voltage applied between the bit line and theword line.
 6. The structure of claim 5 wherein the cell is in a standbymode when a differential potential below the pull-out voltage is appliedbetween the bit-line and the word line.
 7. The structure of claim 2wherein the cantilever beam is coupled to the storage capacitor andmoves from a first position wherein said cantilever beam does not makeelectrical contact with the bit line to a second position wherein saidcantilever beam makes contact with the bit line responsive to a pull-involtage applied between the storage capacitor and the word line.
 8. Thestructure of claim 7 wherein the cell is in a standby mode when adifferential potential below the pull-out voltage is applied between thestorage capacitor and the word line.
 9. The structure of claim 1 whereinthe cantilever beam is electrically connected to the bit line, locatednext to the storage capacitor, and upon actuation, said cantilever beammakes contact with the storage capacitor.
 10. The structure of claim 9wherein the cantilever beam is a lateral beam actuated upwards.
 11. Thestructure of claim 9 wherein the cantilever beam is a lateral beamactuated laterally.
 12. The structure of claim 1 wherein the cantileverbeam is a vertical beam actuated side-to-side.
 13. A method forfabricating a dynamic random-access memory (DRAM) cell, said methodcomprising: forming a nano-electro-mechanical (NEM) relay by: coupling acantilever beam to a bit line of a memory array; wherein said cantileverbeam is oriented parallel to a wafer substrate (laterally) that isactuated downward by making an electrical connection to a storagecapacitor; and activating the cell by applying a high voltage to anelectrostatic actuator to cause the cantilever beam to make anelectrical connection to the storage capacitor, thereby electricallyconnecting the storage capacitor of the cell to the bit line to performeither a read or write operation.
 14. A dynamic random-access memory(DRAM) array structure comprising a plurality of DRAM cells, each cellcomprising: a word line; a bit line; and a nano-electro-mechanical (NEM)relay comprising: a cantilever beam; a storage capacitor; and anelectrostatic actuator coupled with the storage capacitor and verticallyelectrically connected to the word line, wherein said electrostaticactuator is separate from the cantilever beam and applies a high voltageto cause the cantilever beam to electrically connect the storagecapacitor to the bit line to perform either a read or write operation.